Lithium ion batteries (LIBs) provide significant improvements in energy density and cost per watt hour compared to the NiCad and Lithium metal hydride cells that preceded them. Notwithstanding, the manufacturing costs to produce a LIB is cost prohibitive in electric vehicles. Furthermore, the low energy density causes our electronic gadgets to be larger and bulkier than desirable. Recent improvements in the field have attempted to address these drawbacks by increasing the density of solid-state cells.
While cells with lithium metal anodes provide superior energy density, rechargeable cells cannot be constructed with lithium metal anodes because of the risk of dendrite formation during the charge cycle. The dendrite formation during the charge cycle results in short circuits that cause explosion and combustion during ignition of the liquid electrolyte. The liquid electrolyte is a highly combustible organic solvent and does prevent dendrite growth between the anode and cathode. As a result, LIBs are typically made up of intercalation anodes, which allow lithium ions to be inserted into the crystalline structure rather than being plated onto a current collector. Inserting the lithium ions into the crystalline structure reduces the effective energy storage capacity of the anode to less than 10% the theoretical capacity of lithium metal.
Liquid electrolyte also limits a maximum voltage for the battery. Typical liquid electrolytes decompose above four-volts difference between an anode and a cathode, which effectively limits the maximum open circuit voltage of LIBs to about 3.8-volts. Cathode materials that can produce 6 volts against a lithium anode are considered practical, but not usable in cells with liquid electrolyte. The ability to use such high voltage cathodes could increase the energy density of the cells by 50%.
An obvious solution is to use a nonflammable electrolyte that resists dendrite formation, is unaffected by potentials above 6 volts, and possesses ionic conductivities equaling or approaching those of the liquid electrolytes. While ceramics with high lithium ion conductivities meet those requirements, they also have physical and chemical properties that prevent practical implementations. For example, ceramic materials are typically very rigid and brittle. Furthermore, a practical cell is made up of stacks of sub cells, each in turn includes very thin layers of the basic components of an electrochemical cell. Common approaches include constructing a cell by producing the thin layers (<40 μm for the separator) in sheets and assembling them in order. However, the thin layers are fragile and rarely flat, causing a discontinuous contact between individual layers across the meeting surfaces. Applying pressure to the stack of layers tends to improve the contact, but unacceptably increases the risk of fracturing a layer.
Moreover, applying pressure to the stack of layers fails to create an integrated connection between layers, rather it creates pressure contact between two surfaces. Typical battery materials are chemically active causing the contact to react with the surrounding environment. In other words, surface contacts, even between like materials, will be susceptible to increased ionic and/or electrical resistance at the points of contacts.
Other drawbacks associated with a cell with lithium metal anode includes a difficulty in achieving a true hermetic seal around the anode space. Any oxygen or water ingress into the anode space will cause oxidation of the lithium, so a non-hermetic seal reduces the capacity and eventually destroys the cell as oxygen or water leak into the cell. Although, it is clear liquid electrolyte poses significant drawbacks, liquid electrolyte is able to flow into any open space where a lithium atom was oxidized to a lithium ion and moves across the separator to the cathode, to maintain the ionic conductivity throughout the cell. Ceramic electrolyte does not possess this ability. As a result, the conventional approach to using ceramic electrolyte is to create a planar interface between the lithium metal and the ceramic cathode. In this way, only a thin layer of lithium close to the ceramic electrolyte can oxidize and move into the electrolyte. The result is a very big limitation to the energy storage capacity of the anode. Thin film solid-state cells epitomize this drawback because the useable thickness of the lithium metal anode is only a fraction of the metal deposited.
There is a need to address the short comings of current solid-state cell development efforts.